London (tca/dpa) — It’s been billed as the party of all parties, a star-studded four-day celebration of Britain’s longest-reigning monarch for her 96th birthday and 70 years on the throne. But the Platinum Jubilee of Queen Elizabeth II, which has snarled traffic around Buckingham Palace, given Britons an extra-long weekend and will offer up performances by American, Irish and British stars — Diana Ross and Alicia Keys among them — has also underlined the question of what happens next for a monarchy seemingly inseparable from its aging head. As wall-to-wall live coverage began Thursday of the P…